Shown in FIG. 1 is an integrated circuit bipolar junction transistor (BJT). In general a heavily doped buried layer 20 is formed in a silicon substrate 10 and will form the collector contact region of the transistor. An epitaxial layer 30 is then formed on the substrate 10 and the buried layer 20 to form the lightly doped or intrinsic collector region. Two diffusions 40 and 50 are formed in the epitaxial layer 30 to form the base region 40 and the emitter region 50 of the BJT. For a NPN BJT the substrate will be p-type. The buried layer 20 will be doped n+, the epitaxial layer will be lightly doped n-type, the first diffusion region 40 will be p-type, and the second diffusion region 50 will be doped n+. For a PNP BJT the substrate will n-type. The buried layer 20 will be doped p+, the epitaxial layer will be lightly doped p-type, the first diffusion region 40 will be n-type, and the second diffusion region 50 will be doped p+. In normal operation the emitter-base junction will be forward biased and the collector-base junction reversed biased by externally applied voltages. In the BJT, the breakdown voltage of either the emitter-base or collector-base junctions is dependent upon the emitter-base and base-collector doping profiles. The emitter-base profile is limited by tunneling current which if too large can cause a loss of control of the device [Sze, “Physics of Semiconductor Devices,” John Wiley & Sons, pp 96-107]. If the emitter-base junction is designed properly, the tunneling current can be limited so that this is not a problem. The collector-base junction is normally operated in a reverse bias mode of operation. This causes the electric field to be very high at this junction. The peak electric field at the collector-base junction is determined by the lowest value of either the collector doping concentration or the base doping concentration. In a standard bipolar transistor, the base 40 is doped higher than the collector 30, so the peak electric field will be determined by the collector doping concentration.
For a high cutoff frequency, the collector resistance and the collector-base space charge layer must be optimized for a given base width 60 and emitter-base doping profile. In the instant invention it will be assumed that the emitter-base doping profile is fixed. Therefore, it is the collector doping profile which is changed to optimize the transistor performance for improving the breakdown voltage of the common-emitter configuration (BVceo) and the ratio of the cutoff frequency to the maximum frequency (Ft/Fmax) for a given technology. Previously, tradeoffs had to be made in optimizing both BVceo and Ft/Fmax. These tradeoffs involved the collector width 70 and the collector doping levels. An optimum BJT will maximize both BVceo and Ft/Fmax. Such optimization and maximization is not possible under current design methodology. There is therefore a need for an improved BJT that simultaneously maximizes both BVceo and Ft/Fmax for optimum performance.